CDMA is a modulation and multiple access technology based on the spread spectrum communication. The capacity of CDMA-based system is mainly restricted by the interference among mobile stations within the system. As a result, if a signal of the mobile station meets quality requirements of the minimum signal noise ratio (SNR) when it arrives at a base station, the system capacity will be maximized. The purpose of CDMA-based system power control is to maintain high quality communication while minimizing the interference to other subscriber within the same frequency band.
Reverse link power control is also referred to as uplink power control, which adjusts transmission power of a mobile station in real time, so that a signal of the mobile station within a cell just meets the required SNR threshold when it arrives at a receiver of a base station, regardless of the distance from the mobile station to the base station.
The reverse link power control of the WCDMA system includes three aspects: open loop power control, close loop power control and outer loop power control. The open loop power control is used to determine the initial transmission power of the mobile station. The close loop power control, as a main part of the reverse link power control, effects throughout the period in which the communication traffic exists, so as to overcome the fast fading on a radio propagation path. The outer loop power control, as a subsidiary of the close loop power control, adjusts the SNR threshold used in close loop power control according to the quality of the received reverse link signal, so that the power control is directly associated with communication quality instead of merely the improvement of the SNR.
U.S. Pat. No.5,812,938 discloses a method for close loop power control in IS-95 CDMA system. According to the disclosure, the base station periodically measures the SNR of the signal transmitted from the mobile station, compares the measured value with prescribed SNR threshold, generates power control command according to the comparison result, and transmits power control command to the mobile station. The mobile station adjusts its transmission power according to the power control command, so that the SNR of the signal approximates the SNR threshold when the signal arrives at the base station. Since the data at various rates should meet requirements of different bit error rate (BER), it is necessary to set a minimum SNR threshold in respect of each data rate. BER is a common-used measurement of link quality. The lower the BER is, the better the link quality is, and vice versa. Since the actual mobile radio propagation environment is in an ever changing state, and so does the speed of the mobile station, the lowest SNR threshold that meets the link quality requirements will be continuously changed within a certain range. In addition, since it is impossible to find out the corresponding function relation between the SNR threshold and the BER measurement value, the error in SNR measurement will also affect on the control accuracy of the inner loop power control. As a result, it is difficult to control a close loop power based on fixed SNR threshold so as to meet the BER requirements.
A method for controlling reverse link outer loop power in IS-95 CDMA system is disclosed in Journal of China Institute of Communications, Vol. 19, No.10, October 1998, Page 60, entitled “An Uplink Power Control Scheme in CDMA Mobile Communication System”. The method comprises: a base station measuring Frame Error Rate (FER) of receive reverse channel one time per frame; and adjusting the SNR threshold used for close loop power control with different step with respect to different condition one time per frame according to a comparison result between the measured value and a predetermined FER threshold as well as current frame error condition. Since the SNR threshold is adjusted one time per frame, it changes quicker, thereby resulting in a larger variance of base station receive SNR. This will increase system block rate and degrade system capacity. (Please refer to IEEE Transactions of Vehicular technowlege Vol. 47, No. 2, May 1998, “CDMA Cellular Systems Performance with Fading, Shadowing and Imperfect Power Control”).